Stress Intensity Factor Approximations for Cracks Located at the Thread Root Region of Fasteners

Nondimensional stress intensity factors have been numerically determined for a straight-fronted edge crack located at the thread root of a fastener. An influence-function or weight-function approach was used in determining K_I for several fastener sizes: diameters ranging from 1 to 6 in. (2.5 to 15.2 cm) and thread pitches of 4, 8, and 12 threads per inch. Several analytical approximations are used in simplifying the stress analysis problem and the development of the weight function. The results for the nondimensional stress intensity factor, K_I/σ(πa)^1/2, are provided as a function of crack depth, bolt size, and thread pitch. The effects of bolt size and thread form play an important role only when cracks are relatively small. These results indicate that local differences caused by thread form and root radius are limited to crack depths within 2% of the net section. All major contributions to elevation in K_I, caused by the presence of the threads appear to diminish for cracks extending beyond 25% of the minor diameter.